The battery charging function of a mobile phone is closely related to safety in use of the mobile phone. Particularly, variations in voltage of the power grid and abnormal operations of the user may cause the battery charger to output a surge voltage. If the surge voltage surges into the mobile phone during the battery charging process, then a potential safety hazard will result.
Common battery chargers for mobile phones have only a limited self-regulating capability, and generally can only regulate voltage variations within a range of 30 kHz (Kilo Hertz) to 60 kHz. However, in practice applications, variations in voltage of the power grid may take place in some areas, or some users may connect the battery charger to a mobile phone firstly before the battery charger is plugged into a socket.
Both the two cases may incur voltage variations greater than 60 kHz, in which case the battery charger will be deprived of the self-regulating capability and output a surge voltage. For instance, at a moment when the battery charger is plugged into the socket, a surge voltage having a duration of 20 ns (nano seconds) and a peak-peak voltage value (Vpp) of 30 V will be outputted. This surge voltage will surge into the mobile phone to impact the mobile phone, which is very likely to cause damage to or have other adverse effects on the mobile phone.
Referring to FIG. 1, there is shown a charging circuit of a conventional mobile phone. The charging circuit comprises a charging port 101, a capacitor C0, a metal oxide semiconductor (MOS) transistor M0, a diode D0, a resistor R0 and a power management unit (PMU) 102. The charging port 101 has an output terminal connected to a first input terminal of the PMU 102, and the capacitor C0 has one end connected to the first input terminal of the PMU 102 and the other end grounded. The MOS transistor M0 has a source connected to the first input terminal of the PMU 102, a gate connected to an output terminal of the PMU 102 and a drain connected to the resistor R0 via the diode D0; and the resistor R0 is connected to a battery. Moreover, a cathode of the diode D0 is connected to a second input terminal of the PMU, and an anode of the battery B0 is connected to the second input terminal of the PMU 102.
In the aforesaid charging circuit of the mobile phone, no protection circuit against the surge voltage is provided. Consequently, once a surge occurs, the power input, individual control ports and the battery itself of the mobile phone will all be impacted. Although there are over voltage protection (OVP) devices available in the market, the OVP devices have a reaction time of about 5 μs and, thus, cannot provide effective protection against the surge. Moreover, the OVP devices have a high cost.
Accordingly, there is still room for improvement and advancement of the surge protection technologies of the prior art mobile terminal.